The present invention relates to electrochemical cells, and more particularly to spirally wound cells wherein the electrodes and electrolyte are wound around a cylindrical core. The invention is more particularly directed to an improved core structure for such cells. The invention is particularly applicable to spirally wound secondary cells.
In a typical spirally wound cell, a "jelly roll" of the anode, cathode and separator is wound onto a solid core. The anode and cathode are offset with respect to one another in the axial direction of the core, so that the anode electrode extends by itself from one end of the winding and the cathode electrode extends from the other. After the winding is complete, it is placed in a "can" which forms the outer cover of the battery. A liquid electrolyte is then injected to fill the interior of the can, including the space occupied by the porous separator. The can is then sealed and is ready for operation.
Most battery structures of this type include a solid core, and a drawback of such a structure is that heat generated during operation of the cell, particularly during overcharging, is not dissipated very well from the inner part of the cell. Post mortem analysis on failed batteries show heat spots, signs of burning and electrode damage at the core area of the cell. Another drawback is that the cylindrical core is typically of a relatively small diameter and this makes the beginning of winding difficult, particularly if it is desired to use thick electrodes.
A further problem with prior art rechargeable batteries, particularly Li-Ion batteries, is that during overcharging there may be an excessive pressure build-up and to address this it is common to use a pressure release vent designed to rupture at some predetermined pressure. However, due to mechanical instability of the cell casing, it may be that the casing around the vent deforms under pressure and the rupture pressure of the vent may then become unpredictable. It would therefore be desirable to enhance the mechanical stability of the cell casing.
U.S. Pat. No. 3,490,949 discloses "ring-shaped" battery, where, instead of a solid core of relatively small diameter, the structure employs a larger diameter hollow core. The hollow core provides for improved heat dissipation, and at the same time the larger diameter of the core makes winding easier.
A further example of a hollow core battery structure is found in published European Patent Application 0 620 610 A1, wherein further heat dissipation improvements are provided by way of fins extending axially within the hollow core and/or radial heat dissipating fins on the battery lid.
Neither of the cell structures disclosed in the above-cited references addresses the problem of electrolyte filling, the problem of cell pressure release, e.g., during severe overcharging, or the mechanical support given by the hollow core to the cell structure.